Although Leydig cell functions are highly dependent upon pituitry luteinizing hormone (LH), it is becoming increasingly evident that local factors also play important modulatory roles in the responsiveness of this cell. Recent work suggests that arginine vasopressin (AVP), classically considered to be a neurohypophysial antidiuretic hormone, may be one of these local regulatory factors. AVP has been shown to directly inhibit gonadotropin-induced androgen production via specific V1 receptors located on Leydig cells. While circulating levels of AVP are too low to produce this inhibition, my preliminary results demonstrate the presence of an AVP-like peptide in rat tests in amounts sufficient to mediate this inhibitory effect. These findings suggest local production and a potential intratesticular regulatory role of AVP. In these studies, the cell type(s) which contain and produce AVP will be identified and testicular AVP biosynthesis and bioactivity will be studied. the pituitary hormones which regulare testicular AVP content, AVP receptor concentrations and vasopressinase activity will be examined as will developmental changes in these same three parameters. Brattleboro rats, which are devoid of hypothalamo-pituitary AVP, but not teticular AVP, will be used to further study the intragonadal regulatory role of AVP. Finally, in vivo functions will be studied by intratesticular minipump infusions of vasopressor antagonists or purified AVP antibodies. These studies will provide knowledge of the role of testicular AVP and, moreover, will define an AVP system discrete from the well known hypothalamic-neurohypophysial-renal antidiuretic system. In addition, the proposed work will provide a model for elucidating one of many potential intratesticular modulatory systems for androgen biosynthesis.